European Patent 0 636 687 (National Starch & Chemical Ltd.) describes a method for the preparation of styrene/acrylic polymers, optionally including a minor amount of n-dodecyl mercaptan (DDM) by a solution polymerization process involving polymerization, distillation and then neutralization steps. Acrylic acid is preferably present in an amount in the range 50 to 80% by weight, styrene 20 to 50% by weight and DDM up to 1% by weight in the resulting polymer product. It is not, however, possible to make polymers with higher styrene content by this process.
U.S. Pat. No. 5,122,568 (American Cyanamid) describes a process for preparing styrene/acrylic type polymers involving a solution polymerization step followed by simultaneous neutralization and distillation. Not more than 20 weight percent of acid moieties may be neutralized prior to initiation of distillation. The resulting polymers have a weight average molecular weight of at least 30,000 and are useful in the sizing of paper. Polymers having a styrene:acrylic acid monomer ratio of up to 1.4: 1 can be prepared in this way.
U.S. Pat. No. 5,326,843 (Rohm and Haas) describes an emulsion polymerization process for producing alkali-soluble copolymers of methacrylic acid and water-insoluble, monoethylenically unsaturated aromatic monomer such as styrene. The resulting emulsion polymers have weight average molecular weights in the range 2,000 to 40,000 and can include up to about 75% by weight of styrene.
Great Britain Patent 1 107 249 (Johnson) describes a solution polymerization process for preparing styrene/acrylic acid and similar polymers, including use of non-water-miscible solvents such as methyl isobutyl ketone, toluene, etc. The polymers have a number average molecular weight in the range of 700 to 5,000. The mole ratio of styrene to acrylic acid is in the range 3:1 to 1:1. After reaction, residual solvent may have to be removed.